Metal-air cells include one or more air permeable cathodes and a metallic anode that are separated by an aqueous electrolyte. During discharge of a metal-air cell, such as a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. Metal-air cells are often arranged in stacks within a common housing to provide a sufficient amount of power output. The result is a relatively light-weight battery.
Both primary and secondary metal-air batteries have been developed. A rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged back to the atmosphere through the air-permeable cathode. Examples of rechargeable metal-air cells having an anode positioned between two air cathodes are disclosed in U.S. Pat. No. 5,569,551 and U.S. patent application Ser. No. 08/543,558, the disclosures of which are incorporated herein by reference.
Problems common to rechargeable metal-air cells include cell swelling and anode relocation. Anode relocation contributes to capacity loss, operating voltage loss, and may cause an imbalance in current distribution between the cathodes. Although known designs have decreased the problem of anode relocation, further improvements can be made to further decrease capacity loss, operating voltage loss, and imbalances in current distribution between the cathodes.
One phenomenon that contributes to anode relocation is cell swelling. Cell swelling can occur, for example, during the initial discharge cycle of a cell. During a discharge cycle, oxygen is drawn into the cell. When the anode is discharged the volume of the discharge products is roughly twice the volume of the zinc metal. If a cell case is not properly constrained during this reaction, the cell case can swell. As a result, when the electrochemical reaction in the cell is reversed by recharging, the zinc anode can re-form in a distorted space within the cell. This can cause the shape of the anode to become distorted.
While conventional techniques for restraining cells attempt to preclude cell swelling and the resulting anode distortion, some cell swelling and anode distortion can still occur. For example, glues that are used to hold cells often fail due to the forces associated with cell swelling. An additional problem experienced with metal-air cells pertains to the mechanical brackets and spacers that are commonly used to restrain cells. It can be difficult and labor intensive to install certain types of these brackets and spacers.
Accordingly, there is a need for metal-air cells that can be joined in a manner that seeks to preclude cell swelling. Decreased cell swelling will result in decreased anode relocation, and will provide cells with increased power output, without compromising the efficiency and life of the cells.